1. Field of the Invention
The present invention relates to film deposition methods by a magnetron sputtering apparatus, and particularly to improvements in methods of depositing a thin film of metal oxide by a magnetron sputtering apparatus with a mobile magnet for creating a magnetic field moving across a film deposition region.
2. Description of the Related Art
A typical example of depositing a thin film of metal oxide by a magnetron sputtering apparatus is deposition of a conductive thin film of metal oxide on a photoelectric conversion layer of semiconductor in a thin-film solar battery. Particularly, a thin-film solar battery for large power to be installed on a roof of a house, the top of a building and the like is required to have a larger area to provide larger power efficiently at low cost.
In depositing a thin film of a large area, a magnetron sputtering apparatus with a mobile magnet is used more preferably than that with fixed magnets, since if the magnetron sputtering apparatus with fixed magnets is used to deposit a thin film on a fixed substrate of a large area, a large number of magnets must be arranged across the entirety of a large film deposition region and thus the apparatus with fixed magnets becomes complex and expensive. Furthermore, even if a large number of magnets are used, it is difficult to create a uniform magnetic field across the entirety of the large film deposition region, so that a target tends to be unevenly consumed and a film tends to be unevenly deposited. Incidentally, there also exists a magnetron sputtering apparatus with fixed magnets, wherein a substrate is moved relative to the fixed magnets, though such an apparatus with fixed magnets requires a large vacuum chamber and a large vacuum pump to move a large substrate in the chamber and also requires a large installation area therefor.
In a magnetron sputtering apparatus with a magnet for creating a magnetic field reciprocating across a film deposition region, conventionally the magnet typically reciprocates across the entirety of the film deposition region multiple times to provide a uniform quality to a film deposited on a fixed substrate. For example, Japanese Patent Laying-Open No. 10-158833 discloses on page 5, paragraph [0034] that a magnet is exemplarily reciprocated eleven times to deposit an ITO (indium-tin oxide) film having a thickness of 60nm.
However, when a magnetron sputtering apparatus with a mobile magnet is used with the magnet being reciprocated multiple times to deposit a transparent conductive oxide (TCO) layer of metal oxide such as ITO, SnO.sub.2 , or ZnO on a photoelectric conversion layer of semiconductor in a thin-film solar battery, the obtained thin-film solar battery cannot have sufficiently satisfactory characteristics and in particular tends to have a fill factor, as a well-known output characteristic, of insufficient level.
FIGS. 2 and 3 are illustrative, schematic cross sections of thin-film solar batteries each with a TCO layer deposited on a photoelectric conversion layer of semiconductor. In the FIG. 2 thin-film solar battery, successively stacked on a transparent glass substrate 1 are a SnO.sub.2 layer as a first electrode layer 2, a photoelectric conversion layer 3 of semiconductor, and a ZnO layer as a second electrode layer 4. Photoelectric conversion layer 3 of semiconductor includes a p-type sublayer 3p, an i-type sublayer 3i, and an n-type sublayer 3n that are stacked successively. Relatively thick i-type sublayer 3i provides photoelectric conversion, while p- and n-type layers 3p and 3n much thinner than i-type sublayer 3i create an electric field. The sublayers may also be formed of either amorphous semiconductor or crystalline semiconductor.
In the FIG. 3 thin-film solar battery also, first electrode layer 2 of SnO.sub.2 and photoelectric conversion layer 3 of semiconductor are stacked successively on transparent glass substrate 1. The FIG. 3 thin-film solar battery, however, includes a ZnO layer 4a and an Ag layer 5 that are stacked successively on photoelectric conversion layer 3 of semiconductor , as disclosed in Japanese Patent Laying-Open No. 55-108780. ZnO layer 4a acts to enable a second electrode layer to have a high light reflectivity to provide an improved conversion efficiency, and Ag layer 5 acts as the second electrode layer. Ag layer 5 is preferable as a back electrode layer, since it is highly light reflective and highly conductive. ZnO layer 4a between semiconductor layer 3 and Ag layer 5 is much less thick than the FIG. 2 ZnO electrode layer 4, but it can act to realize an enhanced reflectivity of Ag layer 5 and also prevent Ag atoms from diffusing into semiconductor layer 3.
As such, it is known that a thin-film solar battery preferably includes a TCO layer deposited on a photoelectric conversion layer of semiconductor. As has been described, however, if a magnetron sputtering apparatus with a mobile magnet is used with the magnet being reciprocated multiple times to provide a thin-film solar battery with a TCO layer deposited on a photoelectric conversion layer of semiconductor, the obtained solar battery as shown in FIG. 3 can hardly have satisfactory output characteristics and in particular tends to have a degraded fill factor.